1. Field of the Invention
The present invention relates generally to distortion in communication links, and more specifically to non-linear distortions caused by high power amplifiers in communication links.
2. Discussion of the Related Art
Bandwidth efficient modulation techniques are used to increase data rates without increasing bandwidth in a communication link. As efficiency is increased, e.g., by using higher order bandwidth efficient modulations, signal quality becomes more sensitive to nonlinear and linear distortions such that in order to maintain acceptable performance, complex components such as equalizers or linearizers must be added or the communication link suffers reduced range.
Bandwidth efficient modulation formats, such as 16 QAM (16-ary quadrature amplitude modulation) and 64-QAM, become increasingly non-constant in power with more efficiency. Particularly in most wireless and satellite communication systems, these modulation formats are very sensitive to AM (amplitude modulation)/AM and AM/PM (phase modulation) distortions caused by high power amplifiers, such as traveling wave tube amplifiers (TWTAs), at a given transmitter such that symbols at a receiver are not received at the ideal symbol location and may be received as an incorrect symbol resulting in a higher symbol error rate. FIG. 2 illustrates an I,Q plot for an ideal 16-QAM constellation, the X's denoting the ideal symbol location for a given symbol in I,Q space. FIG. 3 illustrates an I,Q plot over time of the 16-QAM constellation that has undergone a nonlinear distortion introduced for example, by a high power amplifier during transmission. The +'s indicating the location of measured individual received symbols over time (illustrated as clustered together). As can easily be seen, the received symbols are distorted from the ideal locations due to the AM/AM and AM/PM distortions. Many of the received symbols are close to the decision threshold lines 202 that the receiver uses to decide which symbol is received, which increases the probability of error in determining which symbols are received, particularly in the presence of other channel related distortions or noise present in the communication link or channel.
One solution to reduce the effects of such nonlinear distortions is to back off the high power amplifier, i.e., operate the amplifier at less than its full power level. However, since there is less transmit power, there is less distance between symbols of the modulation format. Thus, the signal is more susceptible to noise in the communication link. In a satellite communication system, this technique also results in the use of a high power amplifier that can provide more power than is actually used, disadvantageously resulting in extra weight and power on the satellite.
In another solution, the signal is predistorted through the use of another nonlinear circuit inserted in the channel prior to the source of the nonlinear distortions at the transmitter or after the signal is received at the receiver. Fixed versions of these additional circuits are hand-tuned specifically for the source of the distortion, e.g., the high power amplifier, and can not be adjusted for variations in performance due to end-of-life degradation or paths through redundant amplifiers. Adaptive versions of the circuits require complex control algorithms. Particularly in a satellite system, such circuits at the satellite add more weight, power and complexity to the satellite. Additionally, known circuit implementations do not completely cancel non-linearities and often cause more linear distortion; therefore, degrading the channel as much as they improve it.